1. Field of the Invention
The present invention relates to a single sidesealing type metal vapor discharge lamp and, in particular, a single side-sealed type miniature metal halide lamp.
2. Description of the Related Art
Conventionally, high intensity-discharge lamps have been employed for outdoor illumination and factory illumination, but they begin to be employed for indoor illumination for a low-ceiling establishment, such as shops. Of the high intensity-discharge lamps, a metal halide lamp, in particular, is often used, in a miniature form, in the aforementioned field of application in view of its high luminous efficacy and high color rendition. In order to miniaturize high intensity lamps, single endsealed type quartz double tube lamps are now in development because it has been difficult to attain the aforementioned purpose with the use of both end-sealed bulb structure.
This type of lamp has the advantages in that it can be miniaturized since its inner and outer tube are sealed at a single end, and it has an excellent heat resistance because of the outer tube also being made of quartz glass.
FIG. 1 is a diagrammatic view showing an arrangement of an arc tube for a single end-sealed metal halide lamp.
In FIG. 1, arc tube's bulb 4 comprises substantially spherical discharge portion 5 defining discharge region 1 and sealing portion 6 located at one side. Bulb 4 contains a rare gas, mercury and metal halide in predetermined quantities for starting. Within discharge region 1, electrodes 7a, 7b are located in opposed relation and have their end portions connected to metal foils 8a, 8b buried in sealing portion 6. Lead wires 9a and 9b are connected at one end to metal foils 8a and 8b, respectively, and have the other ends associated with an external electrical circuit. Of discharge portion 5, portions 12a and 12b which are defined behind electrodes 7a and 7b are located adjacent to shoulder portions 6a and 6b of sealing portion 6. By incorporating the arc tube within the outer tube of quartz glass it is possible to obtain a metal halide lamp.
When the metal halide lamp is turned ON, an arc is developed across electrodes 7a, 7b. In this case, the arc does not face the inner wall of portions 12a and 12b and, in comparison with the arc facing portion, the inner wall of portions 12a and 12b receives less heat to provide the coolest zone on the inner wall of bulb 4. The coolest zone is further decreased in its temperature due to the absorption of heat by adjacent shoulder portions 6a, 6b. This lowers pressure in the arc tube, determined by a temperature at the coolest zone, and thus exerts an adverse effect over the lamp characteristics, such as the luminous efficacy and color rendition.